Intensified neutral loss tags and use thereof in mass spectrometry

ABSTRACT

Ultrasensitive detection of a chemical substance (analyte) by mass spectrometry is achieved by employing a molecular tag that yields an intense parent ion and then an intense daughter ion in a multi-stage mass spectrometer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Application No. 60/435,179 filed Dec. 19, 2002 entitled, USE OF INTENSITY/INTENSITY TAGS IN MASS SPECTROMETRY, the whole of which is hereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government Support under Contract Number R33 CA84641-03. Therefore, the U.S. Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Mass spectrometry (MS) is a powerful, widely used technique for the qualitative and quantitative analysis of chemical substances ranging from small molecules to macromolecules. In general, it is a very sensitive and specific method, and it comes in many forms, allowing gases, liquid and solid samples to be analyzed. Many of the samples are biomedical or environmental.

Among the major limitations that have held back the general usefulness of mass spectrometry, however, three are relevant here. The first limitation is that the MS signal tends to be analyte dependent, which means that some analytes are much more sensitive than others when detected by this technique. The second problem is background chemical noise, which comes primarily from the sample and often prevents high sensitivity (the ratio of signal to noise). Such noise typically is worse when the amount of analyte is lower or when the analyte is derived from a complex biological or environmental sample as opposed to having its origin as a standard. The third problem, which is related to the first, is that the optimum conditions in the mass spectrometer for high sensitivity tend to depend on the analyte.

Some analytes inherently are more sensitive than others to detection by tandem mass spectrometry because they readily form a gas phase parent ion in the mass spectrometer. For other analytes, the parent ions can undergo a favorable dissociation when activated energetically, as in a collision-induced dissociation step, to form a favorable daughter ion which may be detected relatively free of noise. A relatively intense daughter ion can arise when the compound naturally contains a bond that preferentially dissociates upon energetic activation. An example of this second property is the sensitive detection of a drug containing a labile piperidinyl-allyl bond by MS/MS (Dear et al., 1999). However, many analytes lack either of such favorable properties for sensitive detection by MS/MS.

A common strategy in mass spectrometry to increase signal strength for a poorly-responding analyte is to covalently tag the analyte with a signal-enhancing molecular group. For example, a cationic tag can be employed to enhance the signal for an analyte in an electrospray mass spectrometer or a laser desorportion mass spectrometer including the technique of matrix assisted laser desorption ionization (Zhao et al., 1997). As a second example, (2-hydroxyethyl)trimethylammonium chloride (choline chloride), a saturated compound (lacking double or triple bonds), was used to derivatize fatty acids after the latter were converted to acid chlorides, and the products were detected in a tandem mass spectrometer (Johnson, 2000). Because the choline chloride tag lacked a reactivity group, it was necessary to create an acid chloride reactivity group on the analyte. A comparison of the trimethylammonium and dimethylamine derivatives showed that the former were more prone to detection by neutral loss (of trimethylamine). The application of the method to hydroxy fatty acids was especially complicated by the need to protect the hydroxy group on these acids by acetylation. Additional fragment ions also formed by neutral loss from the hydroxy fatty acids, further compromising the sensitivity. Moderate sensitivity at best was achieved.

Chemical tags have been disclosed with reactivity groups that were reacted with peptides for the purpose of enhancing the formation of sequence-specific daughter ions (Zaia et al., 1995). A complication reported by these authors was that some of the tagged peptides lost a neutral fragment from the tag. These neutral-loss cleavages were considered to be a nuisance since they competed with cleavage along the peptide chain. The latter cleavages were desired since they are the ones that provided sequence information. In most cases, the abundance of the analyte-characteristic daughter ion from neutral loss was less than 1% of the intensity of the parent ion, but in one case it was reported to be 20.9%. Since the relative abundance of the peptide ions from neutral loss was relatively low, even in the latter case, Zaia and Biemann stated, in regard to the ions from neutral loss, “These ions are not sufficiently abundant to detract from the quality of the spectrum.” Of the tags studied, one that minimized neutral loss was recommended. In a review on the subject (Roth et al., 1998), it was similarly stated, “The added chemical derivative group should not fragment during analysis by tandem mass spectrometry, because that fragmentation might complicate the mass spectrum and reduce the intensity of the other fragment ion peaks.” Thus, the best tags for the purpose of enhancing peptide sequencing by tandem MS have been considered to be the ones with minimal fragmentation characteristics. The general use of phosphonium tags to enhance sensitivity for mass spectrometry was also covered in this review.

Thompson et al. (2003) have introduced the concept of a molecular tag termed a “tandem mass tag” (TMT) for quantifying relative amounts of protein-derived peptides in two samples by means of tandem mass spectrometry. The relative quantitation is based on measuring the terminal charged group (sensitization group) of the tag that is released from the tagged peptide upon collision in the collision-induced-dissociation part of the mass spectrometer. It is, therefore, important for the method of Thompson et al. that the released part of the tag is lost as an ion rather than as a neutral fragment, so that it can be detected efficiently in the mass spectrometer. Pairs of tags differing in terms of isotope distribution are used, where each member of each pair of tags has the same overall content of isotopic atoms, but a different location of these atoms. One member of each pair of tags is to be used separately in a non-combined way for the tagging step for each peptide sample to be compared one against the other. After this tagging step, the two samples are combined. The detected ions for relative quantification, which are tag-derived, would be the same for every pair of peptides in the two samples being compared. Thus, these relative quantification ions provide no qualitative information about the peptides, such as their masses. There is also the option of detecting the residual peptide part based on the charge provided to it by protonation.

Finally, aryl sulfates can readily lose a SO₃ neutral fragment in the collision cell of a tandem mass spectrometer. For example, Zhang et al., (1999), observed that the base peak in the tandem mass spectrum of several estrogen sulfates, in which the sulfate is attached to the phenyl ring, came from loss of SO₃ from the parent anion. The sulfo group was similarly observed to be labile in sulfotyrosine peptides analyzed in a tandem mass spectrometer (Rappsilber et al., 2001). However, no tags were employed having an aryl sulfate moiety.

Thus, there is still need for improved analyte tags for mass spectrometry that would consistently provide increased sensitivity. In addition, a category of tags that were not dependent for their performance on characteristics of the analyte would be particularly desirable.

BRIEF SUMMARY OF THE INVENTION

The invention is directed to ultrasensitive detection of a chemical substance (analyte) by mass spectrometry by employing a molecular tag that yields an intense parent ion and then an intense daughter ion in a multi-stage mass spectrometer. Molecular tags according to the invention can be represented as XxyYZ where Z is a reactivity group, which enables XxyYZ to be attached covalently to an analyte A to form product XxyYZ-A; X is a group that bears a charge; x and y are atoms that connect X and Y by a single bond; Y includes an unsaturated group; XxyYZ-A becomes an ion that undergoes predominant cleavage in the gas phase upon energetic activation to form a neutral species Xx and an ion yYZ-A; and the charge on ion yYZ-A from this cleavage is resonance stabilized by distribution between y and the unsaturated group in Y. The structure of the molecular tag according to the invention can lead to ultrasensitive detection of the molecular tag-labeled analyte by formation of a dominating, tag-dissociated, analyte-characteristic daughter ion in high yield, both relative to other ions and on an absolute basis.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims, taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a reaction sequence for using a molecular tag according to the invention in mass spectrometry;

FIG. 2 shows a reaction sequence for the synthesis of certain molecular tags (3, 5 and 6) according to the invention;

FIG. 3 shows a reaction sequence for the synthesis of additional molecular tags (11 and 12) according to the invention;

FIG. 4 shows the detection of 4-aminobiphenyl using molecular tag 13 according to the invention;

FIG. 5 shows the detection of estradiol using molecular tag 14 according to the invention;

FIG. 6 is a trace showing detection by mass spectrometry of 25 fmol of deoxyadenosine-5′-monophosphate labeled with molecular tag 6 according to the invention; and

FIG. 7 is a trace showing detection by mass spectrometry of 500 amol of deoxyadenosine-5′-monophosphate labeled with molecular tag 6 according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a technique for markedly increasing sensitivity in tandem mass spectrometry based on the use of a reagent termed “intensified neutral loss tag” (INLT or INLT tag), which is used to covalently label an analyte of interest. A method for the use of an INLT tag according to the invention is summarized in FIG. 1.

As can be seen in the reaction scheme presented, a sample (10) containing an analyte (A) (12) and (typically) Contaminants (14) is labeled with an INLT of choice (16) to form tagged analyte species TA (18) and tagged contaminant species TC (20). The tagged sample is then subjected to two (or more) stages of mass spectrometry (tandem mass spectrometry). As will be explained in more detail below, not only does the INLT tag according to the invention enhance the formation of a parent ion of the INLT-labeled analyte (TA ion) (22) in the first stage of mass spectrometry (24), but it also enhances the formation of a daughter ion (26) in the second stage (28).

INLT tags according to the invention can be represented as XxyYZ where Z is a reactivity group, which enables XxyYZ to be attached covalently to an analyte A to form product XxyYZ-A; X is a group that bears a charge; x and y are atoms (which may be substituted) that connect X and Y by a single bond; Y includes an unsaturated group; XxyYZ-A becomes an ion that undergoes predominant cleavage in the gas phase upon energetic activation to form a neutral species Xx and an ion yYZ-A; and the charge on ion yYZ-A from this cleavage is resonance stabilized by distribution between y and the unsaturated group in Y. The INLT tag provides ultrasensitivity not only because the X group enhances parent ion formation in the first stage of a two-stage mass spectrometer, but also because the xyY portion of the tag is structured to facilitate loss of the neutral compound Xx upon energetic activation of the parent ion in the second stage, producing a daughter ion bearing the same type of charge as the parent ion. The neutral loss is intensified in the INLT by means of resonance stabilization of the charge on the daughter ion. This specific enhancement of both parent and daughter ion formation can lead to ultrasensitive detection of the INLT-labeled analyte by forming a dominating, tag-dissociated, analyte-characteristic daughter ion in high yield, both relative to other ions and on an absolute basis.

Energetic activation of an XxyXZ-A ion in the mass spectrometer can be accomplished in several ways, e.g., collision with neutral gas molecules (collision-induced dissociation), collision onto a solid surface, and absorption of one or more photons. A diversity of reactivity groups can be used for the Z portion of an INLT tag. These include active esters such as N-hydroxysuccinimide (NHS) esters and carbodiimide activated carboxylic (carboxylic acid or carboxylate) groups. Other examples of reactivity groups are primary amines, hydrazides, acyl halides, alkyl halides including benzyl halides, epoxides, maleimides, imidazole, aldehydes, diazonium, isothiocyanate, sulfhydryl, carbene and nitrene.

In more detail, use of the INLT tag according to the invention increases sensitivity by mass spectrometry in seven complementary and overlapping ways, keeping in mind that sensitivity is the ratio of signal to noise. First, noise is reduced by use of the INLT because many contaminants will be discarded in the first stage of the mass spectrometer where selection of a parent ion takes place. Second, of those contaminants that remain after the first stage, many will be discarded in the second stage of the mass spectrometer since they do not give a fragment ion equivalent in mass to that of the daughter ion created by the selected neutral loss. Third, the ease of selected neutral loss of the Xx fragment using this method means that fragmentation in the analyte portion of the INLT-labeled analyte molecule is reduced. Selected neutral loss incorporates the energy available from energetic activation in the second stage for preferential cleavage of the x-y bond of the tag portion before this energy can break other bonds in the INLT-labeled analyte parent ion. The yield of the selected daughter ion, therefore, is high relative to other ions, giving a mass spectrum dominated by the selected daughter ion. In turn, this means that a low activation energy can be used, which minimizes fragmentation and, therefore, noise from contaminating compounds. Fourth, the high yield of the selected daughter ion from the selected parent ion thereby produces an intense mass spectrum signal in absolute terms for the selected daughter ion. Fifth, both the parent ion and the selected daughter ion are characteristic of the analyte. This means that their masses are dependent on the analyte, which not only is important for identifying analytes, but also for distinguishing the INLT-labeled analyte from other compounds (chemical noise) in the mass spectrometer. Sixth, the method of the invention allows one to control the mass of the parent and daughter ion to increase sensitivity. For example, this can be accomplished by varying the substituents on the ammonium or phosphonium group of an INLT tag carrying those groups. This can also be done, e.g., for carboxylic and sulphate INLT tags, by relying on stable isotopes such as ²H, ¹³C or ¹⁸O. Thus, the method enables the daughter ions to be placed at noise-free masses in the mass spectrum in order to overcome known interferences. Seventh, use of an INLT allows the parent and daughter ions for a small sized analyte to be raised to a higher mass that is more free of background noise in the mass spectrometer.

Intensifying a selected neutral loss in formation of the daughter ion is a key feature of the method of the invention, which, in combination with enhancement of parent ion yield, can lead to ultrasensitivity not just for standard trace analytes, but for trace analytes in real samples. Intensification of a selected neutral loss from an INLT tagged analyte parent ion, in combination with favorable formation of the parent ion, is achieved by setting up resonance stabilization of the charge that arises in the remaining portion of the INLT from the selected neutral loss. This resonance stabilization is based on including an unsaturated moiety in the Y portion of tag XxyYZ in close proximity to the y atom so that the charge that develops on y during the selected breakage of the xy bond during daughter ion formation will be stabilized via conjugated sharing with the unsaturated moiety. For example, choosing a nitrogen atom for x that is made cationic by alkyl, aryl or proton substitution, an alkyl- or hydrogen-substituted carbon for y, and phenyl for Y leads to a benzyl cation as the yYZ-A ion when the xy bond breaks from neutral loss of the Xx group. Other options for Y in this case are ethenyl (forming an allylic cation) and ethynyl (forming a propargyl cation). Choosing an oxygen atom for x, a sulphonyloxy (sulphonate group) (SO₃ ⁻) for X (giving a sulfate), and a phenyl group for Y leads to a phenolate anion as the yYZ-A ion when the xy bond breaks with neutral loss of sulfur trioxide. A carboxylate group (CO₂ ⁻) can also be used instead of a sulphate.

Examples of instruments that can be used to provide tandem MS in the method of this invention include triple quadrupole, ion trap (e.g., quadrupole or ion cyclotron) and time-of-flight/time-of-flight mass spectrometers. An ion trap mass spectrometer is preferred, especially a conventional (Paul) ion trap or a linear ion trap, including ion trap/time-of-flight mass spectrometer or an ion trap/ion trap mass spectrometer. Multiple analytes can be detected together by INLT labeling followed by tandem mass spectrometry, where multiple daughter ions then are detected together.

It is generally preferred to employ an INLT that forms one or more relatively unique neutral products in the dissociation step of the INLT technique, so that the loss of more common neutral products such as H₂O from background parent ions isobaric with the targeted parent ion do not interfere. For example, an unusual neutral product such as triethylamine can be formed from an INLT that contains a (Et)₃N⁺CH₂-phenyl moiety. It is generally preferred to employ an INLT where the ionization and dissociation of the INLT-labeled substance of interest is independent of proton loss or adduction.

Nonisotopic and isotopic forms of an INLT can be used in combination to add further specificity to the measurement. First the analyte of interest is reacted with a mixture of the two tags. Then, after introduction of the reacted sample into the mass spectrometer, the two parent ions, isotopic and nonisotopic, are both selected for energetic activation. If the expected ratio of exclusive daughter ions is not observed for a given analyte, then the presence of an interference or abnormal condition in the mass spectrometer is revealed. Along these same lines, the opportunity to substitute one daughter tag for another to measure an analyte creates an opportunity to vary the mass of the final daughter ion(s) that is observed, until a mass is found where no background interfering ions are present.

One can also use a pair of INLTs that give the same mass for a given parent ion but different masses for the daughter ions to enhance sensitivity or specificity. This can be accomplished by preparing pairs of INLT that both contain the same number of isotopic atoms, yet differ in the substitution sites such that only one member of each pair forms an isotopically-labeled daughter ion. For example, (¹³C² H₃) (CH₃)₂N⁺CH₂C₆H₄R and (CH₃)₃N⁺CH₂C₆D₄R can be used in this way, where R comprises a reactivity group such as an N-hydroxysuccinmide ester. While these two INLTs have the same mass, they would give daughter ions of different masses.

It is attractive to use INLT tags as labels because of their ultrasensitivity to multiplexing in ligand assays, such as immunoassays and hybridization assays. A cocktail of INLT-labeled reagents could be used where each reagent gives one or more parent or daughter ions having a unique mass. To detect these INLT-labeled reagents simultaneously in the case where the parent ions in the detection process differ in their masses, multiple parent ions would be selected in the first stage of the tandem mass spectrometer for dissociation and detection in the second stage.

A major advantage of the INLT technique is that it removes or mitigates the need to tune the ion source and collision conditions to optimize the sensitivity of the method for each analyte. This is especially important in the detection of unknown analytes at trace levels, where the opportunity to tune these conditions may not exist, or be very limited. It is also very important in multi-analyte detection.

The INLT technique can be advantageous for structural characterization, e.g., of nucleotides or peptides, since it can give rise to unique daughter ions in the second stage of a tandem mass spectrometer that, in turn, can be fragmented into structurally informative and unique granddaughter ions in the third or higher stages of tandem MS. Further, since the INLT technique provides daughter ions of high intensity, it can help to minimize the inherent problem of progressively weak signals with each stage of tandem MS.

The method of the invention also can be practiced by taking advantage of the technique of employing a R₃QCX₂-aryl, R₃QCX₂-alkenyl or R₃QCX₂-alkynyl moiety as part of the INLT, where X is selected from H, D, alkyl, alkenyl, alkynyl or aryl. Q is N or P, and at least one R is alkyl, alkenyl, alkynyl or aryl. The alkenyl, alkynyl, or aryl group can be substituted with other groups. The invention further can be practiced with carboxylic (carboxylic acid or carboxylate) versions of INLT tags in which a keto group is located β to the carboxylic group, e.g., where the XxyY part of the tag is HO₂CCH₂CO, HO₂CNHCO or HO₂COCO, and the carboxylic group is in a carboxylate form.

In one form of this invention, moieties such as allyl, benzyl and propargyl are used as part of a molecular group that contains a dissociative-prone bond leading to a carbocation in the second stage of the tandem MS measurement. In each case, the carbocation arises at a carbon which, in turn, is connected to an activating group (namely alkenyl for allyl, phenyl for benzyl, and alkynyl for propargyl). Examples of other activating groups or cores of groups that can be used in the same or similar way are naphthyl and other aryls, indole or imidazole and other heteroaryls, ferrocene or other organometallics, cyclopropyl, ethereal oxygen, conjugated polyalkenyl, allenyl, norbornenyl, norbornadienyl, N═C, N═N, NH—N═C, and related species including combinations of these and the alkenyl, phenyl and alkynyl groups such as styryl. The tendency of these groups to enhance the formation of carbocations can also be increased by adding substituents such as alkoxy or dialkylamino. For example, electron-donating groups (incorporated herein by reference) in ortho or para positions stabilize arylmethyl cations (Goldacre et al., 1949).

It is an option to utilize a high spray voltage when the invention is practiced with electrospray mass spectrometry in order to enhance the formation of an intense parent ion in the first stage of a tandem MS instrument. While the high spray voltage tends to increase the formation of background ions, the net result can be enhanced sensitivity because of the ease of forming a daughter ion in the INLT technique, especially when it entails the co-formation of an unusual neutral product, and especially when a daughter ion is formed at a relatively high mass.

Correspondingly, when the INLT technique is practiced in a laser desorption mass spectrometer including use of matrix-assisted techniques, it is an option to employ a high fluence laser desorption pulse. While a high spray voltage in electrospray or a high laser fluence in laser desorption can decrease sensitivity in ordinary forms of these techniques (because of increased noise), it can increase sensitivity in the INLT technique according to the invention.

The following examples are presented to illustrate the advantages of the present invention and to assist one of ordinary skill in making and using the same. These examples are not intended in any way otherwise to limit the scope of the disclosure.

All chemicals were used as received unless otherwise stated. ¹H NMR was recorded on a Varian 300 MHz Mercury-300 spectrometer.

Referring to FIG. 2, a summary of the reaction sequence used to synthesize specific INLT tags according to the invention 3, 5 and 6 is shown. Commercially available (Aldrich) (4-bromomethyl) phenyl acetic acid was converted to a corresponding NHS active ester and then the bromo atom was replaced with triethylamine, yielding INLT tag 3. Reaction of 3 with N,N-dimethylethylenediamine gave intermediate 4, which in turn was coupled with 2 to give INLT tag 5. Also, INLT tag 6, having an imidazole reactivity group, was then prepared by reacting 5 with histamine. The similar conversion of compound 4 into INLT tags 11 and 12 was accomplished as shown in FIG. 3. As seen, in this case, compound 4 was reacted with the NHS ester of 4-bromobutyric acid to give 11, and the latter compound was then coupled to histamine to give 12. Synthesis of INLT tags 13 and 14 is shown in FIGS. 4 and 5, respectively. On INLT tags 3, 5, 6, 11 and 12 any of the ethyl residues on the terminal triethylammonium group can be replaced by an alkyl (e.g., methyl, propyl or butyl), aryl, alkenyl or alkynyl group.

The specific reaction conditions to produce the indicated INLT tags according to the invention are given below:

4-Bromomethylphenyl acetic acid NHS ester (2)

4-Bromomethyl phenylacetic acid (472 mg, 2 mmol) was dissolved in 3 ml acetonitrile (MeCN) and 2 ml tetrahydrofuran (THF). N-hydroxysuccinimide (NHS) (253 mg, 2.2 mmol) and dicyclohexylcarbodiimide (416 mg, 2 mmol) were added. The mixture was stirred overnight at room temperature. Dicyclohexylurea (DCU) was filtered off, solvents were evaporated. Yield of 2 was 632 mg, which contained a little DCU. The pure 2 can be obtained using the following method: 2 is dissolved in acetonitrile and methanol, then the solution is sucked by vacuum through a fritted funnel, and 2 crystallized out. ¹H NMR (CD₂OCD₃, ppm): 2.87 (s, 4H), 4.06 (s, 2H), 4.66 (s, 2H), 7.39-7.49 (m, 4H).

4-(Triethylamino)methyl phenylacetic acid NHS ester (3)

Compound 2 (850 mg, 2.6 mmol) was dissolved in 20 ml dry, THF. Triethylamine (210 μl) was added. A white solid precipitated instantly. After 15 minutes, the solid was filtered off, and dried under vacuum. Yield of 3 was 1.09 g ¹H NMR (CD₃OD, ppm): 1.42 (t, 9H, J=7.2 Hz), 2.83 (s, 4H), 3.26 (q, 6H, J=7.2 Hz), 4.09 (s, 2H), 4.48 (s, 2H), 7.54 (s, 4H).

2′-[4-(Triethylamino)methyl phenyl acetamido]-ethyl-N,N-dimethyl amine (4)

Compound 3 (854 mg, 2 mmol) was dissolved in 10 ml MeCN. N,N-dimethylethylenediamine (234 μl) and triethylamine (836 μl) were added respectively. The solution was stirred for one hour at room temperature, and then evaporated. Ethyl ether was added, the mixture was shaken, and the ether was decanted. The oily residue was vacuum dried. A mixture of compound 4 and NHS (1:1 mole ratio) was obtained, and was used for further reaction without separation. 1H NMR (CD₃OD, ppm): 1.42 (t, 9H, J=7.2 Hz), 2.35 (s, 6H), 2.56 (t, 2H, J=6.6 Hz), 2.63 (s, 4H, from N-hydroxysuccinimide), 3.25 (q, 6H, J=7.2 Hz), 3.35 (t, 2H, J=6.6 Hz), 3.59 (s, 2H), 4.46 (s, 2H), 7.41-7.52 (m, 4H).

4″-{2′-[4-(Triethylamino)methyl phenyl acetamido]-ethyl-N,N-dimethylamino methyl} phenyl acetic acid NHS ester (5)

The mixture of compound 4 and NHS from the previous step was dissolved in 10 ml dry MeCN. compound 2 (590 mg, 1.9 mmol) was added. A sticky gel was separated from the solvent. After overnight stirring at room temperature, MeCN was decanted. The residue was dried under vacuum and was identified as 5 by proton NMR.

Amine Conjugate of 5 with the Primary Amino Group of Histamine (6)

Compound 5 was dissolved in 5 ml dry dimethylformamide (DMF). Histamine (200 mg, 1.8 mmol) and triethylamine (753 μl) in 5 ml MeCN/water (50/50, v/v) were added to DMF solution. The mixture was stirred for one hour at room temperature, and diluted with acetone. Solvents were decanted. The residue was dissolved again in a minimum amount of methanol and diluted with acetone. Solvents were decanted again. Finally, the residue was purified by reverse phase C2-Si (EM sciences) flash chromatography. After sample was loaded onto the column, the columns was washed by MeCN, then eluted with MeCN/methanol (80/20, v/v). Yield of 6 is 260 mg. ¹H NMR of 6 (D₂O, ppm): 1.20 (t, 9H, J=7.2 Hz), 2.72 (t, 2H, J=6.3 Hz) 2.86 (s, 6H), 2.99 (q, 6H, J=7.2 Hz), 3.24-3.39 9(m, 4H), 3.43 (s, 2H), 3.53 (s, 2H), 3.63 (t, 2H, J=6.3 Hz), 4.16 (s, 2H), 4.24 (s, 2H), 6.88 (s, 1H), 7.12-7.40 (m, 8H), 8.08 (s,1H).

4-Bromobutyric acid NHS ester (10)

4-Bromobutyric acid (3.34 g, 20 mmol) was dissolved in 25 ml methylene chloride and 20 ml acetonitrile. N-hydroxysuccinimide (2.37 g 20 mmol) and dicyclohexylcarbodiimide (4.16 g, 20 mmol) were added respectively. The mixture was stirred for two hours at room temperature. Dicyclohexylurea was filtered off. Solvents were evaporated. The resulting dark yellow oil was subjected to flash chromatography purification using ethyl acetate/hexane/acetic acid (40/60/1, v/v). After evaporation of solvents, a white solid was obtained. The yield of 10 was 4.04 g.

4″-{2′-[4-(Triethylamino)methyl phenylacetamido]ethyl-N,N-dimethylamino}butyric acid NHS ester (11)

Compound 10 (149 mg, 0.56 mmol) and a mixture of 4 and N-hydroxysuccinimide (1:1 mole ratio) (240 mg, 0.47 mmol) in 4 ml dry acetonitrile (distilled over phosphrous pentoxide) were placed in a pressure tube. The tube was heated to 80-90 degree (C) overnight, then cooled to room temperature, and diluted with THF. Solvents were decanted. The residue was dried under vacuum. Yield of 11 was 240 mg.

Amide Conjugate of (11) with the Primary Amino Group of Histamine (12)

Compound 11 (225 mg) was dissolved in 2 ml dry DMF. Histamine (38 mg) and triethylamine (140 microliters) were dissolved in 1 ml acetontrile/water (50/50, v/v). These two solutions were combined and stirred for one hour at room temperature. After concentration by evaporation, the residue was diluted with acetone and a sticky gel precipitated. The mixture was filtered through a fritted funnel packed with white quartz sand. The sand was eluted with methanol. After evaporation of methanol, the residue was dried under vacuum. The yield of 12 was 47 mg.

4-(Sulfonyloxy)benzoic acid NHS ester (13)

p-Hydroxybenzoic acid was converted to 4-(sulfonyloxy)benzoic acid with pyridine sulfur trioxide in pyridine as described (N. B. Desai, V. Rananathan & K. Venktaraman, Raney Nickel Reductions: Part V-A General Method for the Reduction of Quinones to the Corresponding Hydrocarbon Derivatives, J. Sci. Industr. Res., 14B, 1955, 330-334). In turn, this intermediate can be converted into product 13 by reaction with 1,3-dicyclohexylcarbodiimide and N-hydroxysuccinimide, or with 1-[(3-dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide

4-(Sulfonyloxy)benzyl bromide (14)

p-Cresol can be reacted with pyridine sulfur trioxide in pyridine to form 4-(sulfonyloxy)toluene, which in turn can be converted into product 14 by reaction with N-bromosuccinimide.

EXAMPLE 1 Detection of Amino-biphenyl using the Method of the Invention

Compound 13 was coupled to 4-aminobiphenyl analyte in the presence of a water soluble carbodiimide, such as 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. The resulting conjugate can be detected by electrospray tandem mass spectrometry by detecting the phenolate-containing daughter ion produced by collision-induced dissociation of the parent ion with loss of SO₃ neutral. This assay is illustrated in FIG. 4.

EXAMPLE 2 Detection of Estradiol Analyte using the Method of the Invention

Estradiol analyte is detected by derivatization with compound 14 followed by electrospray tandem mass spectrometry, relying on the measurement of the daughter anion formed by loss of SO₃ as the neutral fragment. This example is illustrated in FIG. 5.

EXAMPLE 3 Detection of deoxyadenosine-5′-monophosphate (5′dAMP) using the Method of the Invention

Very sensitive detection of 25 fmol at S/N=50 of an INLT-labeled analyte compound by tandem mass spectrometry (ThermoFinnigan LCQ) is shown in FIG. 6, in which a predominant loss of neutral Et₃N from the parent ion leads to an intense, analyte-characteristic daughter ion at m/z 773.

Two mg of INLT tag 6, 3 mg 5′dAMP, 20 mg EDAC, and 1 ml of 0.2 M MES buffer, pH 6.2 were mixed in a plastic vial. The reaction mixture was kept at room temperature for 4 hr, and 20 μl was injected into HPLC. The product peak (compound 7, the phosphorimidazolide conjugate of INLT 6 and 5′-dAMP) was collected, and the concentration of the collected solution was determined with UV spectrometry at 260 nm, assuming the molar extinction coefficient of product was the same as that of dAMP (1.34e4). A solution of 1×10⁻⁸ M of 7 in methanol:H₂O, 1:1, was infused into an LCQ electrospray mass spectrometer (Thermo Finnigan) over a 2.5 minute period (25 fmol of 7). MS/MS spectra were accumulated over this period relying on continued isolation and collisionally-induced dissociation of M⁺ (874.5 u), forming, through loss of (CH₃CH₂)₃N, daughter ion 773.3 u. The mass spectrum is shown in FIG. 6. S/N (signal to noise ratio) for the latter ion is >50. Source conditions for electrospray: capillary temperature: 250° C.; sheath gas: 20 units; auxiliary gas: 0 units; spray voltage: 4.5; collision energy: 23 units.

EXAMPLE 4 Ultrasensitive Detection of an INLT-labeled Analyte Compound

FIG. 7 further demonstrates the ultrasensitivity of detection of an INLT-labeled analyte compound according to the invention an INLT tag. To obtain this mass spectrum, 0.5 μl of 1×10⁻⁹ M (500 amol) of compound 7 was infused into a tandem electrospray ion trap mass spectrometer (Thermo Finnigan Deca XP), a related, more modern instrument compared to the mass spectrometer used in Example 3. Compound 7 was detected at m/z 773, analogous to its detection in Example 3.

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While the present invention has been described in conjunction with a preferred embodiment, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein. It is therefore intended that the protection granted by Letters Patent hereon be limited only by the definitions contained in the appended claims and equivalents thereof. 

1. A method for detecting an analyte, said method comprising the steps of: covalently labeling an analyte, to produce a tag-analyte molecule, with a molecular tag of the form: XxyYZ, wherein Z is a reactivity group that enables XxyYZ to be attached covalently to analyte (A) to form product XxyYZ-A; Xxy is a group that bears a charge; Y is a group comprising an unsaturated group; x and y are atoms that are connected to each other by a single bond; y is connected to an unsaturated group of Y by a single bond; XxyYZ-A becomes an ion (XxyYZ-A ion) in the gas phase of the first stage of tandem mass spectrometry analysis; and said XxyYZ-A ion undergoes cleavage upon energetic activation in the second stage of tandem mass spectrometry analysis to form a neutral species Xx and a resonance stabilized daughter ion yYZ-A wherein the charge on daughter ion yYZ-A is resonantly stabilized by distribution between y and said unsaturated group of Y; causing said tag-analyte molecule to become a parent tag-analyte ion (XxyYZ-A ion) in the first stage of a tandem mass spectrometer; and causing said parent tag-analyte ion to fragment through cleavage from energetic activation to become a resonance stabilized daughter tag-analyte ion (yYZ-A ion) in the second stage of a tandem mass spectrometer, wherein said daughter tag-analyte ion forms from loss of a neutral fragment (Xx) from said parent tag-analyte ion, wherein said neutral fragment comes from the tag portion of said parent tag-analyte ion and wherein said resonance stabilized daughter ion yYZ-A is predominant relative to any other ion detected from said tag-analyte.
 2. The method of claim 1, wherein said tandem mass spectrometer is an ion trap mass spectrometer.
 3. The method of claim 1, wherein Xxy is a quaternary amine or phosphonium group and Y comprises a benzyl, allyl or propargyl group.
 4. The method of claim 1 wherein Xxy is a sulphate or carbonate group and Y comprises aryl.
 5. The method of claim 1, wherein Z is an imidazole group and A contains a phosphomonoester group.
 6. The method of claim 1, wherein said resonance stabilized yYZ-A ion is a benzylic cation.
 7. The method of claim 1, wherein said resonance stabilized yYZ-A ion is a phenolate anion.
 8. The method of claim 1 wherein neutral species Xx is SO₃.
 9. The method of claim 1, wherein neutral species Xx is a tertiary amine.
 10. The method of claim 1, wherein energetic activation is accomplished by collision of the XxyYZ-A ion with a neutral gas molecule in the gas phase. 